Coded arrangement of photocells mounted on rigid body to determine position thereof

ABSTRACT

A system capable of precisely determining the instantaneous geometrical relationship between two or more elements of a large substantially rigid structure, which elements may nevertheless exhibit small amounts of movement relative to one another. The geometrical relationship is determined by focusing light from a source fixed to one of the elements on an encoding sensor also fixed to one of the elements to thus generate digitally encoded signals representative of the position of the light incident on the sensor.

United States Patent Shepherd [72] lnventor Robert lrving 3,154,68810/1964 250/216 Northrtdge, Calif. 3,243,594 3/1966 Lovell et al.250/211 [21] Appl. No. 546,371 3,311,749 3/1967 Briggs 250/209 [22]Filed 3km. 229, 199766 2,346,031 4/1944 Jones et al. 88/14EH [45]Patented an. 1 I 1 2,934,755 4/1960 Canada 88/14A [73] Assignee TheBunker-Ramo Corporation 2,982,859 5/1961 Steinbrecker 88/ 14A p g- OTHERREFERENCES 2 corpora o aware Fortune" October 1951, pp. 120 123 PrimaryExaminer Ralph G. N ilson [54] CODED ARRANGEMENT OF PHOTOCELLS AssistantExaminer-Martin Abramson MOUNTED 0N RIGID BODY TO DETERMINEAttorney-Frederick M. Arbuckle POSITION THEREOF 10 Claims, 8 DrawingFigs. ABSTRAC A f v system capab e o precisely determining the $5.3] 250/22) instantaneous t i l l ti hi b t t or more isoi F f id 1 39/12elements of a large substantially rigid structure, which eleearc 250/229ments may nevertheless exhibit small amounts of movement r 21 gig/14A,NEH relative to one another. The geometrical relationship is deter- [56]References Cited mined by focusing light from a source fixed to one ofthe elements on an encoding sensor also fixed to one of the elementsUNITED STATES PATENTS to thus generate digitally encoded signalsrepresentative of the 2,952,779 9/1960 Talley 250/203 position of thelight incident on the sensor.

| l LLUMlNATlON as I PHOTOSENSITIVE 52 1 I ELEMENTS i} I I l PATENTEUJAN 12ml 35.55.1285

SHEET 1 OF 2 \O Q $2 \4 STRULTURE STRUCTURE GEOMETRV COMPUTER)NSTRUMENTATIfl 1 \LLMM\NAT\ON PHOTOSENSH'WE ELEMENTE,

I INVISN'I'OR. r Rosa/er hey 4 BY m M A 77'OR/VEY CGEDED ARRANGEMENT FPHOTOCELLS MOUNTED 9N REGH) RUDY T0 DETERMINE FOSETIIGN THEIREUF Thisinvention relates generally to apparatus for measuring small amounts ofrelative movement between portions of a substantially rigid structureand is particularly useful for precisely determining the instantaneousgeometric relationships between elements of large structures, such asantenna structures.

increased interest in tracking of and telemetry from long range vehiclesand investigation of ever fainter radio stars have led to increasedrequirements for accurate angular positioning of large steerableantennas for both pointing and tracking. Pointing involves placing theaxis of an antenna within prescribed angular limits of a given target.Tracking involves the determination of the position of a given target toprescribed angular accuracy. Radar systems normally are con cerned withtracking, but deep space telemetry systems (which are power limited) areprimarily concerned with pointmg.

Several recent examples can be cited where the constructionspecifications for an antenna required angular accuracy of a veryprecise nature. For example, a l-foot-high antenna just completed wasrequired to have a pointing precision of 18 are seconds. In anotherinstallation, a 2l0-foot-deep space antenna required a pointingprecision of approximately l are minute.

With todays technology and available structural materials it isimpossible to produce rigid bodies of the sizes indicated so as to meetsuch angular accuracy requirements. Weight and elasticity inherent inthe structure itself preclude the brute force construction of devices solarge and so rigid. Therefore, it becomes essential to tolerate thesmall amounts of relative rotation and translation between elements ofthe structure and to compensate for such variations in geometry bymodifying any calculations which are based upon the structure sorientation.

in view of the foregoing, it is an object of the present invention toprovide a system capable of precisely determining the instantaneousgeometrical relationship between two or more elements which are subjectto small amounts of movement relative to one another.

Briefly, in accordance with the present invention, the geometricalrelationship between two elements is determined by focusing light from asource fixed to one of the elements on a sensor also fixed to one of theelements. A change in the geometrical relationship between the twoelements is indicated by a change in the position of the light incidenton the sensor.

in accordance with one embodiment of the invention, the source andsensor are located on the same structural element, with a reflectorbeing provided on the other element for focusing the source light on thesensor. In accordance with an alternate embodiment of the invention, thesource and sensor are located on different elements. An importantcharacteristic of all of the embodiments, however, is that they are ableto measure both relative rotation and relative translation between thetwo elements.

Another important aspect of the invention resides in the use of a linearsensor comprised of a plurality of light sensitive elements which arearranged to provide a unique digital output code or" each of severaldifferent positions that the light can assume relative to the sensor.Moreover, the light sensitive elements are preferably arranged to definea Gray Code so as to reduce the likelihood of ambiguities occurringwhere the incident li ht may be slightly skewed relative to the sensororientation.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionwill best be understood from the following description when read inconnection with the accompanying drawings, in which:

l is a block diagram generally illustrating a system in accordance withthe present invention;

FIG. 2 is a structural diagram illustrating a typical antenna structurewith which the invention can beadvantageously utilized;

FIGS. 3(a) through 3(e) illustrate various embodiments of theembodiments of the invention; and F IG. 14 schematically illustrates alinear sensor in accordance with a preferred embodiment of theinvention. J

Attention is now called to FIG. l ofthe drawings, which illustrates ablock diagram of a system: hich' can incorporate the teachings of thepresent invention. More particularly, FIG. 1 illustrates a structure 10which can comprise a large antenna structure (for example, as shown inFIG. 2) including a plurality of elements or structural portions which,due to various factors such as weight, wind, temperature change,etc.,can exhibit translation and rotationalmovement relative to each other.In accordance with the present invention, a structure geometryinstrumentation system 12 is provided for measuring the instantaneousgeometrical relationships between critical portions of the structure 10.Data representing these measurements is supplied to a digital computer14 which utilizes the data to compensate for the geometrical variationsin calculations involving data representing the true position of thestructure relative to some reference, such as a true north or the like.

A typical application of a system in accordance with the invention is inthe control of an antenna structure 10a such as shown in FIG. 2. Theantenna structure 10a is comprised of a tower 18 supporting a parabolicdish 20 having a hyperbolic reflector 22 supported in spacedrelationship therefrom, by structural members 24. As previously noted,in many current applications, it is essential to position the antenna toan accuracy of a few arc seconds. When striving for angular accuracy ofa few are seconds, it becomes apparent that the elasticity of largestructures such as shown in FIG. 2 makes them stretch and bend almost asif they were made of rubber. The support ing tower l8 tilts and twists,servosystems lag and hunt, the parabolic dish 20 folds and flutters, andthe support structure 24 sags and vibrates, all reacting to gravity,wind, temperature changes, and tracking accelerations. To achieve eithertracking accuracy or pointing accuracy, it is necessary to determine theeffect on the position of the radio frequency axis (i.e., the axis ofthe parabolic dish 20) in space of all of these deflections. Fortracking, the output of the shaft encoders (not shown) utilized toindicate the position of the parabolic dish 20 is modified to indicatethe position of the target as the sum of the radio frequency axisposition plus the target offset from this axis. For pointing, the beamposition may be adjusted to the desired locationin space by slewing thehyperbolic reflector 22 or the feed horn 25.

In accordance with the present invention, in order to obtain datarepresenting the instantaneous geometrical relationships between variousportions of the structure shown in FIG. 2, various embodiments of theinvention comprising different basic instrument configurations areprovided, as shown in FIGS. 3(a)-3(e). In FIGS. 3(a)3(e) and in theequations pertaining thereto, the various quantities utilized aredefined as follows:

6, rotation of left-hand unit with respect to an arbitrary index aboutan axis perpendicular to the page, clockwise rotation is positive.

9 rotation of right-hand unit with respect to an arbitrary index aboutan axis perpendicular to the page, clockwise rotation is positive.

L relative displacement of the left-hand unit with respect to theright-hand unit, left unit closer to top of page IS positive.

M linear reading on sensor with respect to an arbitrary index,displacement toward top of page is positive.

R spacing between right and left units.

r focal length of imaging optics.

Initially considering the embodiment of FIG. 3(a), it is to be notedthat a source 26 and a sensor 28 (comprising a left-hand unit 29) arepositioned adjacent each other on a first portion of a structure, with aspherical mirror 30 (comprising the right-hand unit 31) being positionedopposite to the source and sensor on a second portion of the structure.The source 26 comprises a light source which preferably providescoherent light as would be available from a laser. However, the lightsource 26 can bc a noncoherent light source e.g., an incandescent bulb)having a slitted mask 32 interposed between it and the spherical mirror30. As will be seen more clearly hereinafter, it is only essential thatthe source 26 and mask 32 define a sharp line of light for reflection bythe spherical mirror 30 against the sensor 28,

Preferably, the sensor 28 is comprised of a plurality of light sensitiveelements 34 arranges in rows and columns, as is illustrated in FIG. 4.More particularly, FIG. 4 illustrates a linear digital sensor in whichthe six rows represent different degrees of significance of digitalcodes produced by the sensor. More particularly, the bottom row of lightsensitive elements 34 can be considered as being the least significant,the top row as being the most significant, and the intermediate rows, ofcourse, as being of intermediate significance.

In the utilization of the embodiment of the invention shown in FIG.3(a), the source 26 and slitted mask 32 focus a line of light on thespherical mirror 30, which is reflected to the sensor in the form of aline image 38, as shown in FIG. 4. If the quantities L or 9 as shown inFIG. 3(a) are varied, the line image 38 will sweep across thelight-sensitive elements 34, moving parallel to the rows of elements. Itcan be noted that the light-sensitive elements 34 are so arranged inFIG. 4 that for each of several different positions of the image 38, itwill intersect unique combinations of light-sensitive elements. Forexample, when the image 38 is at the extreme left of the diagram, lightwill be incident on the light-sensitive elements in the bottom and toprows of light-sensitive elements. On the other hand, when the image 38is at the extreme right of he sensor, light will be incident only on theelements 34 in the bottom row. Similarly, for each of several difierentpositions (63 different columnar positions are shown in FIG. 2), theimage will overlay a unique combination of rows of light-sensitiveelements 34.

The light-sensitive elements 34 in each row are connected in common toan amplifier. Thus, the light-sensitive elements in the bottom row areall connected to conductor 40, which is connected to'the input ofamplifier 42. Similarly, each of the other rows of light-sensitiveelements are connected respectively to amplifiers 44, 46, 48, 50, and52. It should be apparentthat the output signals provided by theamplifiers 42- 52 are representative of the position of the image 38with respect to the light-sensitive elements 34.

Although the sensor of FIG. 4 has six rows, thereby enabling 63different codes to be defined to indicate 63 different positions of theimage 38, a fewer or greater number of levels could be provided asrequired. As an example, it is pointed out that an eight-level code canprovide precision of better than I in 255, or approximately .4 percent.It is also to be noted that the light-sensitive elements 34 are arrangedin FIG. 4 in accordance with a Gray Code. As is well known in the art,Gray Codes have the characteristic that only one bit of the code changesat a time. This is important in that it permits minor skew misalignmentsof the image 38 to be tolerated without introducing coding errors. It isalso important to note in FIG. 4 that the image 38 is sufficiently longto exceed both the top and bottom rows of light-sensitive elements.Accordingly, the sensor 28 will not be sensitive to vertical movement ofthe image 38. as shown in FIG. 4, but will only be responsive tomovements of the image parallel to the rows of light-sensitive elements.

Returning now to a consideration of the embodiment of FIG. 3(a), itshould be recognized that with the source 26 and sensor 28 located asshown opposite the spherical mirror 30, rotation of the structuralportion on which the source and sensor are mounted (i.e., a variation inangle 9,) will not give rise to an output change from the sensor 28.This should be apparent in as much as changes in the angle 9, willmerely move the light image along the spherical mirror 30, but will notchange the relative position of the image on the sensor 28. However,relative translation between the structural portion on which the sourceand sensor are mounted and the portion on which the spherical mirror ismounted (i.e., a change in quantity L will tend to sweep the image 38along the sensor parallel to the rows of light-sensitive elementsthereof. This will, of course, give rise to a change in the output codeprovided by the sensor amplifiers. Similarly, rotation of the structuralportion on which the spherical mirror 30 is mounted relative to theportion on which the source and sensor are mounted will tend to move theimage 38 along the rows of light-sensitive elements. source and sensorare mounted will tend to move the image 38 along the rows oflight-sensitive elements.

The relationship between the quantities L and 6 and the reading providedby the sensor amplifiers for the configuration of FIG. 3(a) isrepresented by the equation M=2(0 RL The derivation of this equationincludes certain approximations which assure a high level of accuracywhere the arc tana is less than 10 mr.

gent of the quantity In the embodiment of FIG. 3(a), the source 26 andsensor 28 are fixed together at one location, off the axis of aspherical mirror. It will be noted that in the embodiments of FIGS. 3(a)and 3(b), the source and sensor were located on the axis of thespherical mirror 30. In the embodiment of FIG. 3(a), the light providedby the source 26 is focused on a spherical mirror 58, which reflects thelight to a-plane mirror 60, which in turn reflects the light back to thesensor 28. The plane mirror 60 is fixed relative to the spherical mirror30. In this configuration, an aperture is required at the source. Theimage of the aperture is distorted into a linear image at the sensor dueto the astigmatic effect of the spherical mirror of the axis. Curvatureof the spherical mirror is either greater than or less than R, depending upon inclination relative to the sensor axis. The equation forthe configuration shown in FIG. 3(0) is identical to that for theconfiguration shown in FIG. 3(a). It should also be apparent that aconfiguration utilizing a plane mirror with the source and sensor offthe axis of the spherical mirror can be provided corresponding to theconfiguration of FIG. 3(b).

Attention is now called to FIg. 3(d), which illustrates the source 26and sensor 28 fixed together at one location, on the axis of a planemirror 62 fixed at a second location. In this configuration, the lightsource 26 is collimated by collimator 64 and passed through a slittedmask 66, with the slit extending ran transverse to the measurement axisof he sensor. The slit is focused on the sensor by imaging optics 68. Itshould be apparent that utilization of the plane mirror 62 in theembodiment of FIG. 3(d) makes the output of the sensor independent oftranslation L The equation describing the configuration ofFIG. 3(d) isas follows:

In the embodiment of FIG. 3(e), the source 26 is at one location, whilethe sensor 28 is at a second location. A collimator and slitted mask 72are provided in front of the source 26. An imaging lens 74 is providedin front of the sensor 28. It should be apparent that as long as thetranslation L is not excessive i.e., as long as the linear image extendsbeyond the top and bottom rows of light sensitive elements, then theoutput of the sensor 28 in the configuration of Fig. 3( e) will beindependent of the translation L More particularly, the equation ex-This equation is accurate where the arc tangent of the quantity M/R isless than mr. and the arc tangent of the quantity L, /R is also lessthan 10 mr.

Examination of the foregoing embodiments reveals that configurationsshown in FIGS. 3(a), (b), and (0) respond to a combination of relativemotions of the left and right units (transverse to the line of sightbetween them), and the relative rotation of the spherical mirror unit,at the same time being insensitive to rotation of the source/sensorunit. Configurations shown in FIGS. 3(d) and 3(e) respond to angularrotation of both left and right units, but are insensitive totranslation L By choosing a proper combination, e.g., the configurationof FIG. 3(a) and FIG. 3(d), it is possible to determine both rotationand translation of the right unit with respect to the left unit.

From the foregoing, it should be appreciated that a system has beendisclosed herein for determining small amounts of translation androtation of one portion of a structure relative to another. Asindicated, the invention is exceedingly useful in antenna-type and otherstructures where precise positioning is required. The data obtainedrepresenting the amount of rotation and translation can be utilized inthe calculations performed by the computer in order to compensate forthese geometrical variations.

lclaim:

1. In combination with a substantially rigid structure including atleast to portions capable of exhibiting translational or rotationalmovement relative to each other, a system for measuring andquantitatively indicating the positions of said portions relative to oneanother, said system comprising:

sensor means responsive to a light image incident thereon for providingdigitally encoded signals representing the position of said light imagerelative to said sensor means, said sensor means comprising a pluralityof light-sensitive elements arranged in rows and columns, each column ofsaid light-sensitive elements containing elements in a uniquecombination of rows;

means fixedly mounting said sensor means on one of said portions; and

source means fixedly mounted on one of said portions for imaging a lineof light of said sensor means extending substantially parallel to saidcolumns.

2. Apparatus suitable for use with a substantially rigid structureincluding at least two portions capable of exhibiting translational orrotational movement relative to each other for measuring the amount ofsuch movement, said apparatus comprising:

a sensor means adapted to fixedly mounted on one of said portions andresponsive to a radiant energy image incident thereon for providingdigitally encoded signals representing the position of incidence of saidradiant energy image on said sensor means, said sensor means including aplurality of radiant energy detectors arranged in a plurality ofsubstantially parallel rows; and

a source means adapted to be fixedly mounted on one of said portions forimaging a sharply defined line of energy on said sensor means extendingacross said rows substantially perpendicular thereto.

3. The apparatus of claim 2 wherein said plurality of radiant energydetectors are disposed so that said line image intersects a unique setof detectors for each of several different positions extending alongsaid rows.

4. The apparatus of claim 3 wherein all of said radiant detectors commonto a single row are connected in common; and wherein said detectors arearranged in accordance with a Gray code whereby the number of saiddetectors intersected by said line image will differ by only one betweenany two successive positions extending along said rows.

5. The apparatus of claim 2 wherein said source means and said sensormeans are both adapted to be fixedly mounted on one of said twoportions; and including a reflector means adapted to be fixedly mountedon the other of said two portions.

6. The apparatus of claim 5 wherein said reflector means comprises aspherical mirror.

7. The apparatus of claim 6 wherein said reflector means comprises aplanar mirror.

8. The system as defined by claim 1 including means for electricallyinterconnecting all of the light sensitive elements in a single row.

9. The system as defined by claim 1 wherein said sensor and source meansare both mounted on a first of said portions; and reflector meansmounted on a second of said portions.

10. The system as defined by claim 1 wherein said sensor and sourcemeans are respectively mounted on different ones of said portions.

1. In combination with a substantially rigid structure including atleast to portions capable of exhibiting translational or rotationalmovement relative to each other, a system for measuring andquantitatively indicating the positions of said portions relative to oneanother, said system comprising: sensor means responsive to a lightimage incident thereon for providing digitally encoded signAlsrepresenting the position of said light image relative to said sensormeans, said sensor means comprising a plurality of light-sensitiveelements arranged in rows and columns, each column of saidlightsensitive elements containing elements in a unique combination ofrows; means fixedly mounting said sensor means on one of said portions;and source means fixedly mounted on one of said portions for imaging aline of light of said sensor means extending substantially parallel tosaid columns.
 2. Apparatus suitable for use with a substantially rigidstructure including at least two portions capable of exhibitingtranslational or rotational movement relative to each other formeasuring the amount of such movement, said apparatus comprising: asensor means adapted to fixedly mounted on one of said portions andresponsive to a radiant energy image incident thereon for providingdigitally encoded signals representing the position of incidence of saidradiant energy image on said sensor means, said sensor means including aplurality of radiant energy detectors arranged in a plurality ofsubstantially parallel rows; and a source means adapted to be fixedlymounted on one of said portions for imaging a sharply defined line ofenergy on said sensor means extending across said rows substantiallyperpendicular thereto.
 3. The apparatus of claim 2 wherein saidplurality of radiant energy detectors are disposed so that said lineimage intersects a unique set of detectors for each of several differentpositions extending along said rows.
 4. The apparatus of claim 3 whereinall of said radiant detectors common to a single row are connected incommon; and wherein said detectors are arranged in accordance with aGray code whereby the number of said detectors intersected by said lineimage will differ by only one between any two successive positionsextending along said rows.
 5. The apparatus of claim 2 wherein saidsource means and said sensor means are both adapted to be fixedlymounted on one of said two portions; and including a reflector meansadapted to be fixedly mounted on the other of said two portions.
 6. Theapparatus of claim 5 wherein said reflector means comprises a sphericalmirror.
 7. The apparatus of claim 6 wherein said reflector meanscomprises a planar mirror.
 8. The system as defined by claim 1 includingmeans for electrically interconnecting all of the light sensitiveelements in a single row.
 9. The system as defined by claim 1 whereinsaid sensor and source means are both mounted on a first of saidportions; and reflector means mounted on a second of said portions. 10.The system as defined by claim 1 wherein said sensor and source meansare respectively mounted on different ones of said portions.